A. The Field of the Invention
This invention relates to the field of medical imaging. In particular, the invention relates to a digital filmless x-ray cassette.
B. A Description of the Relared Art
This invention relates to medical imaging systems. Film-based x-ray imaging has been in wide use since 1895. Film-based x-ray imaging is a proven and dependable method for capturing x-ray images. However, it fails to meet the emerging needs of modem imaging systems. With the greater use of personal computers in many industries, we see a shift towards modalities that produce digital images. In particular for the medical industry, digital images are becoming more and more important.
Certain proprietary, costly digital imaging systems have been developed, but none provide a low cost, high resolution system that is compatible with the current installed base of x-ray imaging machines.
Before exploring the limitations of present digital x-ray imaging systems, it is worth while to describe the limitations of present film-based x-ray systems.
FIG. 1 illustrates a typical x-ray system. The x-ray machine 101 has an x-ray source 110 and a film cassette 199. The x-ray tube 100 generates an x-ray beam that passes through a collimator 120 through the patient 180 and on to the film cassette 199. The film cassette 199 typically includes an illuminating screen and a light sensitive film. The x-rays cause the illuminating screen to glow producing an image on the film. Where the anatomic structures of the patient 180 block the x-rays, the illuminating screen will receive fewer x-rays. Thus those portions of the film will receive less exposure.
The following describes the typical process of taking an x-ray using a film-based x-ray system. First the film cassette 199 is loaded with the correct illuminating screen and the film. The illuminating screen is a rare earth element that glows when it is irradiated by the x-ray source 110 (emitter). Unfortunately this raises the first problem in taking an x-ray image, that is selecting the correct illuminating screen and film.
Different illuminating screens glow at different levels given the same amount of x-rays and have different response and retention characteristics. Similarly, different types of film will register an image at different rates given the same amount of light from the illuminating screen. Further, the screens and the film must be free of debris. However, every time a new film is placed in the cassette, there is a possibility of contaminating the screen or the film (such as a fingerprint). A contaminated screen or film will cause an obstructed image.
When the film is loaded in the film cassette 199, the cassette must be sealed light tight. Some x-ray cassettes from 3M , Inc. of Minnesota, (e.g., the Tri-max198 x-ray cassette), have two latches and a light tight barrier to help prevent light from entering the x-ray cassette and ruining the image. One of the problems with the present film cassette 199 is that once the film has been placed in the cassette it is possible to either forget that the film has been placed in the cassette or to double expose a given film. The problem lies in that the film cassette 199 does not typically indicate whether a film is inside the cassette or whether that film has been exposed.
The insertion of the film into the cassette is performed in the darkroom. No light can enter a darkroom or the film will become fogged. Similarly, if any light enters a darkroom the rest of the film not in the cassette, but in the darkroom, can also become fogged. The fogged film will then produce lower quality images.
Next the film cassette 199, having the film loaded in it, is inserted in the x-ray machine 101. The film cassette 199 has a front and a back. However, conventional film cassettes 199 can usually fit in either direction. Therefore, it is possible to load a film cassette 199 in backwards into the x-ray machine 101. This will result in an improperly taken x-ray.
One aspect of taking an x-ray is recording information identifying the patient and the date of the x-ray (tag information). The tag information is sometimes included in the x-ray image. In some systems a different processing machine is used to generate the tag information. In other systems the tag information is included as part of the film cassette 199. However, such a system does not typically allow you to collimate, that is reduce the area of x-ray exposure to the patient 180, because the tag information would then be excluded from the x-ray image. Further, identifying the top and bottom of the cassette now becomes important because of where the tag information is a concern. Although a cassette image can be taken on a film that is essentially upside down with respect to the tag image, the office standards will be compromised because of this. Therefore, it is desirable to include tag information in the x-ray image while avoiding the problems discussed above.
An additional problem to inserting the x-ray cassette into the x-ray machine 101 is that the technician needs to actually insert the film cassette 199 every time a new x-ray image is to be captured. This means both the film cassette 199 and the x-ray cassette holder 300 suffer mechanical wear.
Further, some systems provide motorized cassette holders that allow multiple x-ray images to be taken on one piece of film, each x-ray image being next to the previous x-ray image. Such systems are available from Imaging Systems, Inc. in Philadelphia, Pa. However, to simplify construction and increase reliability, it would be desirable to obviate the need of an x-ray motorized cassette holder but still allow multiple images to be taken with the same installed film cassette 199.
After the x-ray cassette has been loaded into the x-ray machine 101, the exposure factor for the x-ray source 110 must be determined. Typically three variables are used to set the exposure factor: the time of the exposure (the more exposure time the more radiation the patient receives), the amps supplied to the x-ray source 110, and the volts supplied to the x-ray source 110. The amperage determines the flow of the x-rays while the volts determine the intensity of the x-rays.
Setting exposure can introduce a number of problems. First, an incorrect exposure time, amps or volts setting can saturate a film image. That is, the exposure is too long and the image is over exposed. Similarly, it is possible to under expose an image. Moreover, overexposure to x-rays poses increased medical risk to the patient. Therefore it would be desirable to supply an imaging system that compensates for incorrect exposure settings on the x-ray source 110.
Next, the x-ray image is taken. The film cassette 199 is removed and taken into the darkroom. The film is then developed in the dark room. Typically the developing involves a number of hazardous chemicals and a relatively expensive developing machine. It is desirable to remove the need to have the hazardous chemicals and the developing machine. Further, it is desirable to provide x-ray images much faster than are provided in film-based x-ray systems. Film developing systems usually take between 90 seconds and 5 minutes or more to produce a single image.
Regarding present digital x-ray systems, these systems tend to be expensive and involve proprietary digital imaging hardware and x-ray machines. Therefore, medical facilities wishing to use digital x-ray imaging systems are required to purchase entirely new x-ray machines with the integrated digital imaging systems. Thus, present digital imaging systems do not provide a digital imaging solution for the large installed base of x-ray machines 101 that presently support film cassettes 199. It is therefore desirable to supply a digital x-ray imaging system that is backwards compatible with the large installed base of x-ray machines 101.
Typically, digital x-ray systems use either charge coupled devices (CCDs) or charged phosphors (CR) to capture digital x-ray images. CCD digital x-ray systems are available from Trophy, Inc. and Schick, Inc. CR digital x-ray systems are available from Sordex-Findent, Inc. and Fuji, Inc.
CCD systems directly capture a portion of the x-ray image. CCD systems include a relatively small CCD sensor (adequate for intra-oral applications, for example, but not to replace large film sizes). It is desirable to have a direct capture digital x-ray cassette that has external dimensions approximately equal to a large size standard film cassette and has an active sensing area comparable to the size of the corresponding film for that large size standard film cassette. Further, because the sensor size is small, to capture images of the complete area of interest in some anatomic structures, additional exposures need to be taken. This has the undesirable effect of increasing the radiation dosage received by the patient.
CR systems have a sensing area approximately the same as the film that they replace. However, CR systems require a scanning of the phosphor after an image has been captured on it. This is somewhat analogous to scanning processed film images. This additional scanning step processes the image captured on the phosphor and turns it into a digital x-ray image. CR systems therefore require more time to capture the digital x-ray image than direct capture systems, such as CCD systems. Therefore, it is desirable to have a direct capture digital x-ray system. Also, present CR systems are integrated with the x-ray tube and the x-ray machine generator, thereby increasing the capital equipment costs of the CR systems. As noted above, it is desirable to reduce the capital equipment costs of the digital x-ray capture system by using already installed x-ray machine.
An example of an x-ray diagnostic apparatus using radiation detectors rather than film is described in U.S. Pat. No. 4,188,537 (hereinafter referred to as Franke). Franke describes an apparatus where, instead of having a film holder for the carrier and film for the radiation receiver, the apparatus has a plurality of radiation detectors which are transducer means for measuring radiation intensity. However, Franke has a number of drawbacks. In particular, Franke does not teach a radiation detector that is capable of being used in present x-ray machines. That is, the Franke radiation detector does not have external dimensions that would enable the radiation detector to be used in an x-ray machine that accepts standard x-ray film cassettes. Additionally, Franke does not describe how the radiation detector would be powered if the detector were to be used in a standard x-ray machine. Franke does not describe how images from the detector could be easily transmitted from the detector to the display device if the detector were to be used in a standard x-ray machine. Further Franke does not describe how to begin capturing an image when the radiation detector is used in a standard x-ray machine. Thus, Franke has a number of limitations.
Another example of an x-ray diagnostic apparatus using radiation detectors instead of film is described in U.S. Pat. No. 4,878,234 (hereinafter referred to as Pfeiffer et al.). Pfeiffer et al. discloses a system where the x-rays cause a scintillation layer to emit visible light. The visible light is transmitted, via a light transmitter(s), to one or more CCD sensors. The sensors the provide digital signals corresponding to the light. One drawback of the Pfeiffer et al. is the need to include the light transmitter(s). The light transmitters increase the size of the radiation detector, reduce the amount of light transmitted to the detectors, and increase the complexity of manufacturing the radiation detector. Like Franke, Pfeiffer et al. does not teach a radiation detector that is capable of being used in place of a standard x-ray film cassette.
Another example of an x-ray diagnostic apparatus using radiation detectors instead of film is described in U.S. Pat. No. 4,823,369 (hereinafter referred to as Guenther et al.). Guenther et al. uses an amorphous silicon image recorder having a scintillation layer applied thereto, or coupled to the image recorder via a fiber-optic plate. As with the other inventions, Guenther et al. does not describe a radiation detector that can be substituted for a standard x-ray machine film cassette. Additionally, the scintillation layer is taught to be a layer formed on the image recorder or coupled to the image recorder by a fiber optic plate. Where the scintillation layer is formed directly on the image recorder, this arrangement does not make clear how one would be able to change the scintillation layer without having replace the relatively expensive image recorder. Additionally, forming the scintillation layer on the image recorder introduces one more manufacturing step in the processing of the image recorder. Where the scintillation layer is coupled to the image recorder by the fiber optic plate, the fiber optic plate increases the size of the x-ray detector and increases the cost of manufacturing the image detector.
Therefore, it is desirable to have a digital x-ray imaging system that does not suffer from the above-noted problems with present x-ray systems including present digital x-ray systems.